The present invention relates to a method of imaging flowing fluids, such as human blood, using nuclear magnetic resonance (NMR).
An example of a conventional NMR display method includes a sequence whereby first to fifth periods are employed to obtain proton nuclear spins in human blood as blood flow information, as shown in FIG. 1.
In Period No. 1, a high frequency RF magnetic field RF(1), 90.degree. RF pulse, is applied together with a first magnetic field gradient G.sub.z (1) to the sample, with second and third magnetic field gradients G.sub.x and G.sub.y being zero. Upon application of the fields RF(1) and G.sub.z (1) to the sample, nuclear spin in a sample slice having a certain thickness is selectively excited, depending upon the frequency of the RF field RF(1). The thickness of the slice can be varied by changing the bandwidth of the RF pulse or the amplitude of the field gradient G.sub.z (1). The first field gradient G.sub.z (1) has an orientation orthogonal to the slice plane, and the field gradients G.sub.z and G.sub.x are orthogonal to each other.
In Period No. 2, in order to excite nuclear spins coming into the slice plane in the third period, a wait time period is provided in the second period, during which the phase of nuclear spin excited in the first period becomes disordered due to non-uniformity of magnetastatic field.
In Period No. 3, a 90.degree. RF pulse RF(3) is again applied together with the first field gradient G.sub.z (3). In this case, the amplitudes of G.sub.z (3) and G.sub.z (1) are made equal to each other. Further, RF(1) and RF(3) are the same in frequency and bandwidth, and the second and third field gradients are zero. As a result, the nuclear spins of protons in blood flowing into the slice plane during the waiting time period of the second time period and the nuclear spins of protons already existing in the slice and whose magnetization is being recovered are excited.
In Period No. 4, the phase of the spins in the z-direction is ordered by applying the first field gradient G.sub.z (4) and, simultaneously, phase modulation is introduced by applying the third field gradient G.sub.y (4). Upon application of the second field gradient G.sub.x (4), reception of a spin echo signal in a subsequent Period No. 5 is readied.
In Period No. 5, a spin echo S(5) is received under the second field gradient G.sub.x (5) and a receiving gate signal R(5).
In order to constitute an image, the above sequence is repeated with the magnitude of G.sub.y (4) being varied. The image constitution is realized by using the so-called "spin warp" method as disclosed in British Pat. No. 2,079,946.
The signal magnitude of respective image elements is thus in proportion to the amount of blood flowing thereinto and the amount of protons whose magnetization is being recovered.
Therefore, since the signal magnitude of the image thus obtained contains components of protons already existing in the slice plane, it is difficult to display only information concerning the fluid flow. Particularly, in order to obtain information concerning flowing fluid the flow rate of which is low, it is necessary to make the waiting time period, i.e., the interval between two 90.degree. RF pulses, sufficiently long. Since the recovery of magnetization of nuclear spin residing in the slice plane becomes large during that period, the resultant data is not always satisfactory.